In cellular mobile communication systems Direct Sequence Code Division Multiple Access (DS-CDMA) is an often used technology. User data are modulated on different PN codes unique to each user channel and transmitted in slots. In the UTRA wideband-CDMA (W-CDMA) systems for example a frame consists of 15 slots. In a Frequency Division Duplexing (FDD) CDMA system different frequency bands are foreseen for uplink and downlink communication, whereas in a Time Division Duplexing (TDD) CDMA system the same frequencies are used for both uplink and downlink. Some time slots in the TDD system are used for downlink and some for uplink transmission.
For fast acquisition in TDD mode a downlink synchronisation channel (SCH) is transmitted by each base station. The SCH consists of two subchannels, the primary and the secondary SCH. The slot synchronisation is derived from the pSCH (Primary Synchronisation Channel). For terrestrial purposes, the SCH is located in only two slots of the frame structure. In each of the two slots there is an active part containing a PN code of a certain length and a part where no synchronisation signal is present. To find the pSCH, a matched filter is used. By ‘matched filter’ is meant a FIR filter matched to the PN code applied in the synchronisation slot. The matched filtering operation is equivalent to calculating a cross correlation between the received signal and the PN code sequence contained in the matched filter. When a correlation peak is found, one has to wait for the second peak occurring 7 or 8 slots later.
This procedure is fast but not robust for two main reasons. To find one of the two correlation peaks it is necessary to set a threshold and at least one of the correlation peaks (the two correlation peaks are relative to the two pSCH present in a frame) has to exceed the threshold. But if one or more noise peaks exceed the threshold or if the average noise is too high to be able to set a correct threshold, it is not possible to find the correct synchronisation without using a dwelling procedure. Secondly, to use the dwelling procedure, it is necessary to store in a RAM the matched filter output for the complete frame duration and then to look for the two maximum values. This means in TDD mode it is necessary to have a RAM 15 times bigger than in FDD mode, which makes this solution too hardware expensive.
Present invention is related to the subject matter of EP application no. 03250482.1 “Programmable Acquisition Module For Multi-standard CDMA Based Receiver”, where a module is disclosed suitable for the acquisition of several kinds of codes.
The present invention aims to provide a flexible and robust procedure for a fast acquisition of the slot edges of TDD W-CDMA signals in burst mode.